In Austrian Pat. No. 290,962, and in U.S. Application Ser. No. 364,190, filed May 25, 1973, there is described a process for the decaffeination of coffee. In accordance with this process a pressure vessel is charged with coffee which can be raw coffee having a water content of 9%, i.e. `dry` coffee. Water which is additionally present in the pressure vessel is used for moistening coffee (enlarging the water content of the raw coffee above the normal water content of about 9%). Supercritical carbon dioxide at a pressure of 120 to 180 atmospheres is passed through the coffee in the pressure vessel at a temperature of about 40.degree. to 80.degree. C. The supercritical carbon dioxide is circulated through the coffee in the pressure vessel with a circulation pump. The supercritical carbon dioxide after passing through the coffee and removing the caffeine is then passed through a second pressure vessel containing active charcoal which removes the caffeine from the carbon dioxide.
In that procedure, moistening of the coffee in the pressure vessel is effected by the circulating wet supercritical carbon dioxide. As a result the coffee, which swells due to the uptake of water, becomes deformed and packs together making it difficult to empty the containers. An additional problem is that on scaling up the procedure it is very difficult in large pressure vessels to pump supercritical carbon dioxide at a uniform flow rate through the whole flow cross-section of a large quantity of coffee. Flow channels, in which the gas flows very rapidly, and zones where the rate of flow is minimal, are invariably formed. By suitable design and modification of the pressure vessel it is possible to reduce this unfavorable situation but it cannot be completely eliminated. As a result the coffee in a particular charge is decaffeinated to different extents.
The amount of caffeine which can be taken up by a given amount of circulating supercritical carbon dioxide, and which is also carried into the adsorption container, is dependent, among other things, upon the residual caffeine content of the beans.
As a result, during decaffeination the gas which streams through the flow channels--and this is frequently the largest fraction--carries only a small amount of caffeine to the active charcoal.
In order to reduce the average caffeine content in a charge to approximately 0.05%, it is necessary to increase the time of treatment of 20-25 h. An increase in the total circulation does not lead to any significant reduction in the length of time of treatment.
After decaffeination the wet coffee is dried in the same apparatus. This has turned out to be a disadvantage because the beans do not contract to their original volume and this is disadvantageous for further processing.